1. Field of the Disclosure
The present disclosure relates generally to catalyst support materials and catalyst materials. The present disclosure relates more particularly to catalyst support materials and cobalt catalyst materials including such support materials, and their uses in Fischer-Tropsch processes.
2. Technical Background
The Fischer-Tropsch process can be used for the conversion of synthesis gas (“syngas,” a mixture of H2 and CO) into liquid and/or solid hydrocarbons. The syngas can be made from a variety of feedstocks (e.g. natural gas, associated gas and/or coal-bed methane, biomass, residual oil fractions and coal). Fischer-Tropsch processes are conducted in a reactor in the presence of a suitable catalyst at elevated temperature and pressure to form paraffinic compounds ranging from methane to high molecular weight compounds comprising up to 200 carbon atoms, or, under particular circumstances, even more. Catalyst materials generally include an active component (e.g., a metal, often provided in the form of an oxide) supported on a catalyst support, which can be a porous refractory oxide such as alumina or silica. The support material can provide a high surface area upon which the active component can be dispersed and a pore network through which the reactant gases can diffuse in and the reaction products can diffuse out. The integrity and durability of the supporting material in the reaction conditions are very critical parameters for the use of the catalyst.
Catalyst materials become less active over time, via a variety of mechanisms. For example, the catalyst can be poisoned by a number of different species including, for example, sulfur, sodium, nitrogen or carbon containing compounds, all of which de-activate the catalyst. Moreover, the dispersion of the metal or metal component may decreases over time. Also, sintering and agglomeration of the support particles reduces the surface area of the support and consequently the activity of the catalyst. Catalyst materials can also break into smaller pieces, especially in fluidized processes, and become entrained in the gaseous effluent. Accordingly, catalyst materials are replaced periodically in order to maintain acceptable product yield.
Fischer-Tropsch processes can be carried out in a variety of types of reaction system; the type of reaction system will dictate the form of the catalyst material. Reactions carried out in fixed-bed reactors can be performed using catalyst pellets, made, for example, by conventional methods such as tableting and extrusion. Currently, the slurry bubble column reactor is commonly used for Fischer-Tropsch processes. Such processes use much smaller catalyst material particles, often substantially spherical in shape and made by spray-drying. The fluidized nature of processes conducted in slurry bubble column reactors requires the particulate catalyst used therein to have a relatively high mechanical strength in order to survive the many interparticle collisions. Moreover, Fischer-Tropsch synthesis in a slurry bubble column reactor exposes the catalyst material to high temperature, high water partial pressure, and acidic species; these features further affect the durability of the catalyst material.
Alumina is a conventional material for use as a support for cobalt-based Fischer-Tropsch catalyst materials, especially those used in slurry bubble column reactors. The alumina is typically provided in a transition form, such as gamma-alumina or theta-alumina, and formed in a shape suitable for the particular reaction system, spray-dried microspherical particles in the case of the slurry bubble column reactor. The active metal cobalt is loaded onto the alumina by conventional methods such as impregnation of a cobalt-containing precursor and calcining to form cobalt oxide. It has been found that under the conditions in a slurry bubble column reactor, the transition alumina support can be rehydrated to boehmite and dissolve in the Fischer-Tropsch synthesis products. This can cause deactivation of the catalyst material, difficulties in catalyst/product separation, deactivation of downstream catalysts and contamination of the products.
Thus, there remains a need to further improve the stability of catalytic support materials and catalytic material used in Fischer-Tropsch processes, especially for use in slurry bubble column reactors.